Hello,
If you are looking for one to exactly replace the one in your 1974 A/S... my guess is you will need to find a used one in good condition either from someone who parts out Airstreams, or on ebay, etc. Many folks here simply fabricate their own using the measurements from the one they are removing... can't say I have ever seen showers, etc. but you can always ask!
In case you haven't checked out these vendors (don't know if they have showers... but do have many parts for vintage 'streams...) here are a couple that do a lot of business with folks here.
Vintage Trailer Supply - Vintage travel trailer parts and supplies!
Airstream Innovations at Inland RV Center, Inc. (951) 734-8130
Good luck!
Mrs. NorCal Bambi traveling in S Tardis ~ from the Great State of Jefferson